The present invention generally relates to an ion extraction system for devices utilizing grids to accelerate and focus ion beams, such as ion thrusters and ion sources. More particularly, this invention utilizes small hole accel grid (SHAG) optics, which minimizes the emission of neutral atoms with a three-electrode system and allows high ion extraction current densities at low net extraction voltages. A particular feature of the invention is the construction of the accel grid that has the advantages of reducing the contamination of surfaces downstream of the electrode system through accel grid sputtering and is easier to fabricate and use than a conventional SHAG electrode.
Present ion extraction systems use two or three electrode systems with either standard hole accel grids, having an open area approximately 50% that of the aperture of the screen grid, or small hole accel grids, having an open area as low as 10% of the aperture of the screen grid. In the latter case, the hole sizes are slightly larger than the individual ion beamlets. The state-of-the-art system utilizes small hole accel grid (SHAG) optics with a three-electrode system comprising a screen grid, the accel grid and a decel grid. A neutral highly ionized plasma upstream of the electrodes is typically held at a high positive voltage of several hundred to several thousand volts. On the other hand, the accel grid is held at a negative voltage of several hundred to several thousand volts, while the decel grid is held at zero or ground potential. The electric field set up by the screen grid and the accel grid extracts an ion beam from the plasma source and focuses it through the accel grid hole. The function of the decel grid is to minimize the beam spread or divergence after the ion beam passes through the accel grid. The small aperture size of the accel grid minimizes the emission of the un-ionized atoms from the discharge plasma, which in ion thrusters maximizes the thruster performance, and in ion sources minimizes the propellant gas flow, thus minimizing the vacuum system pressure.
One disadvantage of this electrode system is the possibility of contamination by sputtered accel grid material. As the ion beam passes through the accel grid aperture, charge-exchange between the high energy ions and the neutral atoms drifting through the aperture can occur, producing a low energy ion which strikes the accel grid. In a three-electrode system, these charge-exchange ions strike and sputter the grid on the barrels of the holes. Although most of the sputtered grid material will be contained within the grids and discharge chamber, some of the material is emitted and can coat surfaces downstream of the grids. Another shortcoming of the electrode system is the difficulty in making the SHAG electrodes. The usual method has been to use a blank plate at the accel grid and let the ion beamlets etch each hole. This process is fairly tedious and can take 24 hours or longer with fairly thick accel electrodes. If the electrode assembly is disassembled and reassembled, perfect realignment is practically impossible so that the electrode system will have to be operated for a time before the ion impingement on the accel grid is reduced to a negligible value.